摘要:

Methacrolein (MACR) is an abundant multifunctional carbonyl compound with high
reactivity in the atmosphere. In this study, we investigated the hydroxyl radical initiated
oxidation of MACR at various NO/MACR ratios (0 to 4.04) and relative humidities (< 3% to
80%) using a flow tube. Meanwhile, a box model based on the Master Chemical Mechanism
was performed to test our current understanding of the mechanism. In contrast to the
reasonable predictions for hydroxyacetone production, the modeled yields of formaldehyde
(HCHO) were twice higher than the experimental results. The discrepancy was ascribed to
the existence of unconsidered non-HCHO forming channels in the chemistry of CH₃C(=CH₂)OO^., which account for approx. 50%. In addition, the production of hydroxyacetone
and HCHO were affected by water vapor as well as the initial NO/MACR ratio. The yields of
HCHO were higher under humid conditions than that under dry condition. The yields of
hydroxyacetone were higher under humid conditions at low-NO_x level, while lower at
high-NO_x level. The reasonable explanation for the lower hydroxyacetone yield under
humid conditions at high-NO_x level is that water vapor promotes the production of
methacrolein nitrate in the reaction of HOCH₂C(CH₃)(OO^.)CHO with NO due to the peroxy
radical-water complex formation, which was evidenced by calculational results. And the
minimum equilibrium constant of this water complex formation was estimated to be
1.89 × 10⁻¹⁸ cm³/molecule. These results provide new insights into the MACR oxidation
mechanismand the effects of water vapor.

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